What is electrical anharmonicity?
Electrical anharmonicity refers to a term in the potential
energy of a crystal which is linear in the externally applied
electric field amplitude, and quadratic in lattice displacement
coordinates. As such, it gives a dielectric polariztion which is
quadratic in the lattice coordinates. Such an interaction term of
lattice and external field allows
the direct absorption of photons by two-phonon states of the
crystal.